Technical Field
This disclosure relates to an internal current limiter or current interrupter used to protect a battery in the event of an internal short circuit or overcharge leads to thermal runaway. In particular, it relates to a high energy density rechargeable (HEDR) battery with improved safety.
Background
There is a need for rechargeable battery systems with enhanced safety which have a high energy density and hence are capable of storing and delivering large amounts of electrical energy per unit volume and/or weight. Such stable high energy battery systems have significant utility in a number of applications including military equipment, communication equipment, and robotics.
An example of a high energy density rechargeable (HEDR) battery commonly in use is the lithium-ion battery. A lithium-ion battery is a rechargeable battery wherein lithium ions move from the negative electrode to the positive electrode during discharge and back when charging. Lithium-ion batteries can be dangerous under some conditions and can pose a safety hazard. The fire energy content (electrical+chemical) of lithium cobalt-oxide cells is about 100 to 150 kJ per A h, most of it chemical. If overcharged or overheated, Li-ion batteries may suffer thermal runaway and cell rupture. In extreme cases this can lead to combustion. Also, short-circuiting the battery, either externally or internally, will cause the battery to overheat and possibly to catch fire.
Thermal Runaway:
If the heat generated by a lithium ion battery exceeds its heat dissipation capacity, the battery can become susceptible to thermal runaway, resulting in overheating and, under some circumstances, to destructive results such as fire or violent explosion. Thermal runaway is a positive feedback loop wherein an increase in temperature changes the system so as to cause further increases in temperature. The excess heat can result from battery mismanagement, battery defect, accident, or other causes. However, the excess heat generation often results from increased joule heating due to excessive internal current or from exothermic reactions between the positive and negative electrodes. Excessive internal current can result from a variety of causes, but a lowering of the internal resistance due to separator short circuit is one possible cause. Heat resulting from a separator short circuit can cause a further breach within the separator, leading to a mixing of the reagents of the negative and positive electrodes and the generation of further heat due to the resultant exothermic reaction.
Internal Short Circuit:
Lithium ion batteries employ a separator between the negative and positive electrodes to electrically separate the two electrodes from one another while allowing lithium ions to pass through. When the battery performs work by passing electrons through an external circuit, the permeability of the separator to lithium ions enables the battery to close the circuit. Short circuiting the separator by providing a conductive path across it allows the battery to discharge rapidly. A short circuit across the separator can result from improper charging and discharging. More particularly, improper charging and discharging can lead to the deposition of a metallic lithium dendrite within the separator so as to provide a conductive path for electrons from one electrode to the other. The lower resistance of this conductive path allows for rapid discharge and the generation of significant joule heat. Overheating and thermal runaway can result.
Overcharge:
In a lithium-ion battery, useful work is performed when electrons flow through a closed external circuit. However, in order to maintain charge neutrality, for each electron that flows through the external circuit, there must be a corresponding lithium ion that is transported from one electrode to the other. The electric potential driving this transport is achieved by oxidizing a transition metal. For example, cobalt (Co), from Co3+ to Co4+ during charge and reduced from Co44+ to Co3+ during discharge. Conventionally, Li1−xCoO2 may be employed, where the coefficient χ represents the molar fraction of both the Li ion and the oxidative state of CoO2, viz., Co3+ or Co4+. Employing these conventions, the positive electrode half-reaction for the lithium cobalt battery is represented as follows:LiCoO2⇄Li1−xCoO2+xLi++xe−
The negative electrode half reaction is represented as follows:xLi++xe−+xC6⇄xLiC6 
The cobalt electrode reaction is reversible only for x<0.5, limiting the depth of discharge allowable. Overcharge leads to the synthesis of cobalt (IV) oxide, as follows:LiCoO2→Li++CoO2+e−
Overcharge is irreversible and can lead to thermal runaway.
What was needed was an internal battery feature for preventing overcharge. What was needed was an internal current limiter that could limit the rate of internal discharge resulting from an internal short circuit, including a short circuit of the separator.